1. Field of the Invention
The present invention relates to a sulfonium salt, a resist composition, and a patterning process.
2. Description of the Related Art
In recent years, as LSI advances toward a higher integration and a more rapid processing speed, finer pattern rules are being requested; and in this trend, a far ultraviolet lithography and a vacuum ultraviolet lithography are promising for the next generation fine patterning technologies. Especially, a photolithography using an ArF excimer laser beam as a light source is an indispensable technology for an ultrafine patterning process with the size of 0.13 μm or less.
The ArF lithography started to be used partially from manufacturing of a device with the 130-nm node, and then it became a main lithography technology from the 90-nm node device. As the lithography technology for the next 45-nm node device, a 157-nm lithography using a F2 laser was initially considered as a promising technology; however, a delay in development thereof due to several problems was indicated. Accordingly, an ArF immersion lithography emerged rapidly because by inserting a liquid whose refractive index is higher than air, such as water, ethylene glycol, and glycerin, between a projection lens and a wafer, the number of aperture (NA) of the projection lens therein can be designed to be 1.0 or more thereby attaining a high resolution (for example, see Non-Patent Document 1); and this is now in the stage of practical use. For this immersion lithography, a resist composition not readily eluting into water is being requested.
In the ArF lithography, in order to avoid deterioration of a precise and expensive optical material, a highly sensitive resist composition that can express a sufficient resolution with a small exposure dose is requested; and in order to realize this, the most generally used method is to select each ingredient having a high transparency at the wavelength of 193 nm. As to a base resin for example, polyacrylic acid and a derivative thereof, norbornene-maleic acid anhydride alternating copolymer, polynorbornene, a ring-opened metathesis polymer, a hydrogenated ring-opened metathesis polymer, and the like have been proposed; and these bring about a certain level of results in enhancing a transparency of a resin.
In recent years, a negative tone resist by an organic solvent development as well as a positive tone resist by an alkaline development has been receiving an attention. In order to resolve a very fine hole pattern, which cannot be achieved by a positive tone, by a negative tone exposure, a negative pattern is formed by an organic solvent development using a positive resist composition having a high resolution. In addition, a study to obtain a doubled resolution power by combining two developments of the alkaline development and the organic solvent development is going on.
As to the ArF resist composition for the negative tone development by an organic solvent, an existing positive ArF resist composition can be used; and patterning processes thereof are disclosed in Patent Document 1 to 3.
In order to catch up the rapid fine patterning trend in recent years, development of a resist material as well as a process technology is progressing day by day. Various photo-sensitive acid generators have been studied; and a sulfonium salt of a triphenylsulfonium cation and a perfluoroalkane sulfonate anion is generally used. However, a generated acid of a perfluoroalkane sulfonic acid, especially perfluorooctane sulfonic acid (PFOS), has concerns about low degradability, biological concentration, and toxicity. And therefore, its use in the resist composition becomes difficult, so that currently a photo-sensitive acid generator to generate perfluorobutane sulfonic acid is being used; however, when this generator is used in the resist composition, it is difficult to achieve a high resolution because of a high diffusibility of the acid generated therefrom.
To solve this problem, various kinds of partially fluorinated alkane sulfonic acids and their salts have been developed; and a photo-sensitive acid generator having 2-acyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonic acid has been developed because of its characteristics including easiness to introduce a substituent group that can control such properties as solubility into a resist solvent, stability, and diffusibility (see Patent Document 4). Meanwhile, in patent document 4, photo-sensitive acid generators to generate α,α-difluoroalkane sulfonic acids by photo-exposure, specifically di(4-tert-butylphenyl)iodonium=1,1-difluoro-2-(1-naphthyl)ethane sulfonate and photo-sensitive acid generators to generate an α,α,β,β-tetrafluoroalkane sulfonic acid are mentioned as conventional technologies; however, although in both generators can lower the fluorine-substitution rates, degradability thereof is insufficient because there are no degradable substituents such as an ester structure; and in addition, there are problems such that molecular design to change the bulkiness of the alkane sulfonic acid is limited, and that a starting fluorine-containing material is expensive.
As to the cation, a triphenylsulfonium cation is widely used; however, this cation has a drawback of a large absorbance at the ArF exposure light (193 nm) to lower the transmittance in the resist film thereby sometimes leading to a low resolution. Accordingly, in order to obtain a high sensitivity and a high resolution, a 4-alokoxy-1-naphthyltetrahydrothiophenium cation and the like have been developed (see Patent Document 5); and a resist composition in combination thereof with components including a resin having a plurality of acid-labile groups is disclosed (see Patent Document 6).
However, as the circuit line width is becoming narrower so rapidly that effect of the contrast deterioration due to acid diffusion is becoming even more problematic in the resist material; and therefore, under the current situation, the sulfonium salts as mentioned above cannot satisfy the required resist performance. This is because the pattern size gets close to the diffusion length of the acid, causing specifically deterioration of the mask reliability and the pattern rectangularity, inhomogeneity of a fine line pattern (line width roughness, or LWR), and the like.
As a resist pattern with a high resolution is demanded in recent years, improvements not only in lithography properties typically represented by a pattern form, contrast, roughness, and the like but also in a defect (surface defect) of a resist pattern after development become necessary more than ever. The defect herein means all the troubles observed from directly above the resist pattern after development, for example, by a surface defect observation apparatus (trade name of KLA, manufactured by KLA-Tencor Corporation). The foregoing troubles include problems after development such as a scum, a bubble, a dust, and a bridge among resist patterns.